Part I
It has previously been proposed to store energy in the form of a liquified gas, such as liquid nitrogen. This liquid or its natural counterpart, liquid air, can be stored and transported in a well-insulated vessel. Energy may be recovered for use from this liquified gas by three simple steps:
1. Pressurization of the liquid to a high pressure by pumping.
2. Vaporization of the cold liquid and heating of the gas from the ambient environment.
3. Expansion of the gas from high pressure to atmospheric pressure in an appropriate engine expander to do useful work.
As noted above, the foregoing has been previously proposed, but the results are so poor that it is of little use for ordinary transportation. A heavy load of liquified gas is required for any reasonable range in an automotive application, and the operating cost is unacceptably high. However, the system does have a number of favorable features including low pollutant levels, and the fact that liquid hydrocarbon fuels are not directly consumed.